The popularity of mobile wireless communications has resulted in the development and use of many new and different wireless communication specifications, techniques and standards. Examples of some current standards are: time division multiple access (TDMA), code division multiple access (CDMA), orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA). The various wireless communication standards enable simultaneous operation of multiple wireless communication sessions involving voice communications, data communications or any type of digital payload.
The many different communication standards operating at different radio frequencies and bandwidths typically require transceiver architecture to be developed specifically for different communication standards. Thus each system will use different hardware architecture.
There are developments to try to overcome the problem of dedicated hardware architecture for current and future wireless systems by using software radio, where the radio frequencies and bandwidths can be modified through software changes on the same hardware. Software radio systems aim to process waveforms as sampled digital signals using software as much as possible, rather than requiring specialised analogue hardware for each processing function. With improvements in processor capacity, functions that are traditionally performed by hardware can be implemented in software.
Current software radio implementations utilise parameterized digital functions such as digital signal mixers, filters, interpolators and decimators that allow signals having a high carrier frequency transmitted over the air to be converted into baseband signals for digital signal processing for the various wireless communication techniques such as TDMA, CDMA, OFDM or OFDMA.
All the above wireless communication techniques maximize transmission over the air through quadrature modulation, where two transmission channels separated by a 90° phase shift are provided, in-phase and quadrature channels (IQ channels). Signals or data transmitted over the two IQ channels are combined before further transmission at higher frequency. The advantage of IQ channels is that two different sampled signals or data can be transmitted sequentially and almost simultaneously over single higher frequency channel by taking advantage the periodicity of oscillated signal through cosine oscillation which can be phase separated perfectly and naturally by 90° through sine oscillation as illustrated in FIG. 1.
The most common method in broadband digital communication system is to convert a signal in the Radio Frequency (RF) or Intermediate Frequency (IF) in a channel from and to the baseband signal directly, illustrated in FIG. 1, where the signal center of frequency is at 0 Hz allowing efficient digital signal processing algorithms to be applied for signal correction and recovery before proceeding to higher layer of communication level. The analogue transmitter circuit in FIG. 1 converts the signal from baseband to higher frequency in stages until it reaches the desired transmission frequency at the transmit antenna 9. The inverse operation is performed for the analogue receiver circuit 10 that converts RF and IF signals starting from the frequency received at the antenna 19 in stages until it reaches the baseband.
Imperfections in analogue circuit implementations specific to the quadrature mixer, produce offsets and errors in the frequency mixers 2 and 11 and the oscillators 3 and 12. For example, these components are commonly influenced by high circuit temperature, thermal noise, and implementation flaws from design and materials used. The offsets and errors generated from the mixers and the oscillators affect the quality of the combined modulated signal from in-phase (I) and quadrature (Q) signal paths as shown in Equation 1 for the transmit mixer output 8, Equation 2 for the receive mixer output 13 and Equation 3 for the receiver mixer output 14. Where xI is the signal for the I-path, xQ is the signal for Q-path, εT is the transmit gain error produced by the imperfect transmit mixer, εR is the receive gain error produced by the imperfect receive mixer, φT is the transmit oscillation phase offset produced by imperfect transmit oscillators and φR is the receive oscillation phase offset produced by imperfect receive oscillators.Y8=(1+εT)xI cos φT+(1−ε)xQ sin φT  (1)Y13=(1+εR)xI cos φR+(1−εR)xQ sin φR  (2)Y14=(1+εR)xQ cos φR+(1−εR)xI sin φR  (3)The imperfection of analogue circuitry produces imperfection of quadrate modulation which, in turn, results in radio frequency signal degradation such as amplitude, phase and frequency offsets to the signal.
A prior art approach to this problem is to implement quadrature modulation digitally through digital mixers, digital cosine and sine oscillation, perfect attenuation digital filters, interpolators and decimators to sample the digital signals correctly at different stages of the signal frequencies from high frequency to baseband and vice versa as illustrated in FIG. 2, which shows a typical digital front-end radio implementation currently used in commercial communication applications.
The errors and offsets of the analogue quadrature mixers can be eliminated through digital implementation as illustrated in FIG. 2 using digital IF sampling method. Implementation of this method provides the term digital front-end radio implementation. The digital transmit quadrature mixer 20 implements digital multiplication 21 as mixer, digital oscillator 22 or numerical control oscillator (NCO) that generates cosine and sine waveform digitally and a digital adder 23. This architecture uses single digital-to-analogue converter (DAC) 28 which is one of the architecture's main advantages. The digital receiver quadrature mixer 29 implements digital multiplication 30 as mixer and digital oscillator 31. Likewise, the receiver architecture uses single analogue-to-digital converter (ADC) 38.
In FIG. 2 the number of analogue-to-digital converters (ADC) and digital-to-analogue converter (DAC) can be reduced to single ADC and DAC but with higher design specification and tolerance requirements due to the high frequency digital sampling rate. The implementation of digital IF sampling module can be very costly due to the complexity of numerical controlled oscillator (NCO) which generates digital cosine and sine oscillation and operates at sampling frequency of multiple times of IF signal frequency. In addition, two stage filters have to be implemented i.e. one is for pulse shaping to smooth digital signals into a natural analogue signal representation form, and another for the interpolator or decimator which increase or decrease sampling rate respectively. Due to this complexity and high cost the use of digital front-end for software radio systems is only feasible for implementation on base stations and access points. The costs involved are prohibitive for providing digital font-end for software radio in terminal or subscriber stations.